chowder, If you weren't into flowering I might suggest a phosphorous deficiency with the red stems, but I've had some suggest that this is a natural process for some strains while in flowering. One of mine had the same thing and I asked around and that's the info I was given. But mine wasn't exhibiting the yellowing and necrotic spots as yours is. So I did some checking and found this and the pic to go along with it. Leaves look a lot like yours. Hope this helps

Nitrogen Deficiencies

Plants will exhibit lack of vigor, slow growth and will be weak and stunted. Quality and yield will be significantly reduced. Older leaves become yellow (chlorotic) from lack of chlorophyll. Deficient plants will exhibit uniform light green to yellow on older leaves, these leaves may die and drop. Leaf margins will not curled up noticeably. Chlorosis will eventually spread throughout the plant. Stems, petioles and lower leaf surfaces may turn purple.